1. Field of the Invention
The present invention relates to a laminated phase retarder adapted for improvement in viewing angle characteristic of a liquid-crystal display device such as a vertical alignment (VA) type liquid-crystal display.
2. Description of the Related Art
A phase retarder having main refractive indices nx, ny and nz controlled in three directions including two in-plane directions x and y concerning oblique viewing directions, and one normal direction z is required for forming a liquid-crystal display device exhibiting excellent display quality in all azimuths while compensating for birefringence of a liquid-crystal cell. Particularly a phase retarder having three-direction main refractive indices satisfying the relation nx greater than ny greater than nz is required for forming a VA type or OCB type liquid-crystal display device.
As the phase retarder having main refractive indices nx, ny and nz controlled, two types of phase retarders have been heretofore known. The first type phase retarder is a laminated phase retarder formed from uniaxially stretched films which are laminated on each other so that directions of in-plane slow axes of the uniaxially stretched films intersect each other perpendicularly. The second type phase retarder is a monolayered phase retarder formed from a polymer film laterally or biaxially stretched by a tenter.
The former laminated phase retarder, however, has a problem that the laminated phase retarder can be hardly mass-produced because films high in thickness accuracy are required as well as the laminated phase retarder is bulky due to lamination of two films. On the other hand, in the latter monolayered phase retarder, the range of the retardation value obtained is narrow. When the retardation value in a thicknesswise direction is remarkably larger than that in a normal direction, at least two monolayered phase retarders must be laminated in the same manner as in the laminated phase retarder in order to obtain a required retardation value. The problem of bulk still occurs. Moreover, there is also a problem that the producing process is complicated because it is difficult to produce the phase retarder consecutively.
An object of the invention is to provide a phase retarder which is thin and excellent in manufacturability and which can be used for forming a liquid-crystal display device such as a VA type liquid-crystal display device improved in view angle characteristic to obtain high contrast.
According to the invention, there is provided a laminated phase retarder having: a laminate of a retardation layer A made of a thermoplastic resin exhibiting positive birefringent characteristic and a retardation layer B constituted by an oriented solidified layer of cholesteric liquid crystal exhibiting a selective reflection wavelength range not larger than 350 nm; wherein Re of the laminate is not smaller than 10 nm and Rthxe2x88x92Re of the laminate is not smaller than 50 nm on the basis of light with a wavelength of 590 nm when Re and Rth are given by Re=(nxxe2x88x92ny)xc3x97d and Rth=(nxxe2x88x92nz)xc3x97d in which nx and ny are in-plane main refractive indices, nz is a thicknesswise refractive index, and d is a layer thickness. There is also provided a polarizing member having: a laminate of a laminated phase retarder defined above; and a polarizer. There is further provided a liquid-crystal display device having: a liquid-crystal cell; and a laminated phase retarder or a polarizing member defined above and disposed on at least one of opposite surfaces of the liquid-crystal cell.
According to the invention, a material such as liquid crystal for forming a retardation layer can be formed with good production efficiency by a coating method without necessity of lamination of films, so that a high-quality laminated phase retarder excellent in reduction of thickness can be obtained with good manufacturability. The laminated phase retarder can be used for improving the viewing angle of a liquid-crystal cell greatly.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.